The semiconductor industry continues to develop lithographic technologies that are able to print ever-smaller integrated circuit dimensions. Modern photolithography typically uses a laser light source, also known as a laser system, to provide very narrow band light pulses that illuminate a mask in order to expose photo-resistive material on silicon wafers, also known as substrates.
Some of the most common systems currently used in photolithography are deep ultraviolet (“DUV”) light systems. DUV light is generally defined to be electromagnetic radiation having wavelengths of between about 5 and 250 nanometers (nm) and is produced by certain types of excimer lasers (argon-fluorine or “ArF,” and krypton-fluorine or “KrF”). To accurately mass produce semiconductors, these systems must be highly reliable and provide cost effective throughput and reasonable process latitude.
The substrates are typically held in devices known stepper-scanners, or simply scanners. Advances in semiconductor device technology continue to place increasing demands on the performance characteristics of both the laser light sources and scanners, requiring continuing improvements in the precision and speed of operation of these devices.
As is known in the art, a sensor in the scanner may periodically communicate the laser light parameters desired to achieve a desired dosage of laser light energy for use in the photolithographic process to the laser light source. In turn, the laser light source can then generate the appropriate laser light and output it to the scanner.
It will be apparent to one of skill in the art that it is not only desirable but very important to be able to control the amount, or “dose,” of DUV light energy being applied to a particular item being treated, such as a semiconductor wafer. The dose is generally defined as the weighted sum of energy delivered to the substrate over a number of consecutive pulses of light generated by the laser. For example, typically a specified amount of DUV light energy, sometimes referred to as a “target dose,” will be required to accomplish a given task, such as curing a layer of photoresist, on a semiconductor wafer as part of the manufacturing process. In order to obtain consistent results across different wafers, it will be desirable to apply the same amount of DUV light energy to each wafer, to as great a degree of accuracy as possible.
There are a number of issues that may arise in providing accurate dose control. The lasers used typically must fire some number of laser pulses to reach stable operation, and thus it can take time for the generated laser light to reach a stable operating point after the desired parameters have been received from the scanner. Noise and other disturbances in the laser source can make it difficult to accurately generate the laser light at the desired energy level. There is also often a tradeoff between stability and performance.
What is needed is an improved way to quickly and accurately control a dose of DUV radiation generated by a light source that is robust against unknown variations in the system gain.